1. Field of the Invention
The present invention relates to a throttle device for an internal combustion engine, more particularly relates to a bearing structure of an electronic control type throttle control device for controlling an opening degree of a throttle valve in accordance with an amount of operation of a throttle by a drive motor or other actuator.
2. Description of the Related Art
As shown in FIG. 4, in the related art, there has been proposed a throttle device for an internal combustion engine provided with a throttle valve for controlling an amount of intake air to the internal combustion engine, a throttle shaft 102 forming a shaft part of the throttle valve 101, a throttle body 104 operably holding the throttle valve 101 in a tubular shaped bore wall 103, a slide bearing 107 press fit inside a first bearing support 105 provided so as to stick out to one side from the outer circumference of the bore wall 103 and rotatably supporting a first sliding part of the throttle shaft 102, a gear side slide bearing 108 press fit inside a second bearing support 106 provided so as to stick out to the other side from the outer circumference of the bore wall 103 and rotatably supporting a second sliding part of the throttle shaft 102, a plug 109 for sealing an open side of a first shaft insertion hole 111 formed in the first bearing support 105, and an oil seal 110 for sealing an open side of a second shaft insertion hole 112 formed in the second bearing support 106. Further, there is known a throttle device for an internal combustion engine forming a closed-bottom boss, for affixing ball bearings rotatably bearing the two ends of the throttle shaft holding and affixing the throttle valve, separately from tubular shaped boss wall forming an intake passage inside the plastic throttle body (for example, see Japanese Unexamined Patent Publication (Kokai) No. 11-173227 (pages 1 to 6 and FIG. 1 to FIG. 9)).
There is also known a throttle device for an internal combustion engine forming the plug 109 shown in FIG. 4 by a material with a good slidability and forming it integrally with the slide bearing 107 for the purpose of reducing costs by supporting one end of the throttle shaft holding and affixing the throttle valve by a ball bearing and bearing the other end of the throttle shaft by a cap-shaped plane bearing (integral plug-type slide bearing) (for example, see the specification of U.S. Pat. No. 5,758,977 (pages 1 to 5 and FIG. 1 and FIG. 2) and Japanese Unexamined Patent Publication (Kokai) No. 2001-289068 (pages 1 to 9 and FIG. 1 to FIG. 7)).
The integral plug-type slide bearing carried in the throttle device for an internal combustion engine described in the specification of U.S. Pat. No. 5,758,977 and Japanese Unexamined Patent Publication (Kokai) No. 2001-289068 is open at one end for insertion of the shaft and forms a closed-bottom cup shape at its other end. The integral plug-type slide bearing is press fit at the outer circumference of its tube inside the inner circumference of a bearing support of the throttle body. Further, the tubular inside circumference of the integral plug-type slide bearing receives the throttle shaft and bears the other end of the throttle shaft so as to enable sliding and rotation. Due to this, the integral plug-shaped slide bearing not only has the function of a slide bearing, but also the function of preventing entry of dust from the outside to the inside of the bore wall (intake passage side) or the function of preventing leakage of a fluid (air, liquid) from the inside of the bore wall to the outside.
However, in the throttle device for an internal combustion engine described in the specification of U.S. Pat. No. 5,758,977 and Japanese Unexamined Patent Publication (Kokai) No. 2001-289068, since the integral plug-type slide bearing is press fit at the outer circumference of its tube inside the inner circumference of the bearing support of the throttle body, a high dimensional precision is required for the inner and outer diameters of the throttle valve, throttle shaft, bore wall of the throttle body, bearing support of the throttle body, and tube of the integral plug-type slide bearing.
The reason is that if there is a large variation in these dimensions, the clearance between the outer circumference of the disk of the throttle valve and the inner wall of the bore wall of the throttle body would become larger when rotational movement of the throttle valve is stopped by a full close stopper etc. at the full close position (when the throttle valve is fully closed) and the amount of leakage of the intake air will increase when the throttle valve is fully closed. Due to this, the idling speed of the internal combustion engine will become higher than the target value, so the problem will arise of the inability of normal idling control.
If moving the full close position of the throttle valve too much to the closing side so as to solve this problem, that is, to reduce the amount of leakage when the valve is fully closed, before the rotational motion of the throttle valve is stopped by the full close stopper etc. at the full close position, the outer circumference of the disk of the throttle valve will interfere with the inner circumference of the bore wall of the throttle body and lock and therefore the problem will arise of the inability to stop the throttle valve at a predetermined full close position.
Further, if enlarging the ring-shaped clearance between the outer circumference of the sliding part of the throttle shaft and the inner circumference of the tube of the integral plug-type slide bearing so as to absorb the variation in the inner and outer diameters and other dimensions of the throttle valve, throttle shaft, bore wall of the throttle body, bearing support of the throttle body, and tube of the integral plug-type slide bearing, the rotational displacement of the sliding part of the throttle shaft due to shifting of the throttle shaft will increase and abnormal wear will occur at the inner circumference of the tube of the integral plug-type slide bearing. Due to this, if the sliding part of the throttle shaft sticks or other problems arise, the valve will lock and the opening degree of the throttle valve will no longer be able to be controlled.
Further, if forming the throttle body and the throttle valve integrally by plastic, the dimensional precision or parts precision will become poorer due to molding shrinkage etc. compared with machined parts, so the problem will arise of the precision of assembly becoming very poor if press fitting the outer circumference of the tube of the integral plug-type slide bearing in the inner circumference of the bearing support of the throttle body.
Therefore, even if forming the boss holding the ball bearing separately from the bore wall of the throttle body so as to improve the precision of formation like in the throttle device for an internal combustion engine described in Japanese Unexamined Patent Publication (Kokai) No. 11-173227, it will be necessary to consider not only the variations in inner and outer diameters and other dimensions of the throttle valve, throttle shaft, bore wall of the throttle body, bearing support of the throttle body, and tube of the integral plug-type slide bearing, but also precision of assembly of the boss in the bore wall of the throttle body, so the above problem is still not solved.